function y = restrict(v);
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nf = length(v)-1;
nc = nf/2;

y = zeros(nc+1,1);
y(2:nc) = (v(2:2:nf-2)+2*v(3:2:nf-1)+v(4:2:nf))/4;
y(1) = 0;  y(nc+1) = 0;


